This invention generally relates to methods for dry bag cold isostatic pressing, and more particularly to methods utilizing dry bag cold isostatic press moulds having improved punches for sealing the mould ends of such so as to produce a compact having a reduced elephant foot effect at the ends of the compact.
Metal powder, such as tungsten, is typically disposed in a cylindrical elastic mould and radially compacted. An apparatus for performing such radial compaction operations is well known in the art as a dry bag cold isostatic press. See "Isostatic Pressing Technology", edited by P. J. James, Applied Science Publishers, New York, Chapter 4, "Tooling for Cold Isostatic Pressing", pp. 91-119, incorporated herein by reference, dry bag cold isostatic pressing is sometimes herein referred to as dry bag pressing.
In dry bag pressing, an elastic bag or mould is fixed within a pressure vessel. The elastic mould has at least one open end which is sealed with the pressure vessel so that the fluid pressure medium within the vessel cannot enter the mould interior. The elastic mould is made from a material which does not chemically react with either the powder or the pressure medium, and readily releases from the powder compact. A material having a high resistance to wear from the powder is also desirable. Mould materials that can be used include natural rubber, neoprene, polyvinyl chloride, butyl, nitride, silicone, and preferably urethane.
In one method of dry bag pressing, a cylindrical elastic mould is used that is open at both ends, and has a void space, such as, for example, a cylindrical void therein. Sealing means for the open mould ends are provided by wear resistant metal punches. The punches are located and restrained by the yoke of the press, and guided into the bag by wear resistant bushes mounted in the pressure vessel. The top punch is removed and powder is charged into the void space in the mould completely filling the void space between the sealing means. The powder is charged into the mould by means well known in the art for providing a fill density that does not allow settling of the powder prior to compaction. A powder fill density of at least about 5 grams per cubic centimeter, and preferably about 6 grams per cubic centimeter is adequate. One method for providing such a fill density comprises pouring a small amount of powder in the mould followed by tamping of the powder, and repeating this procedure until the void space is filled.
The top punch is engaged in the mould and a fluid pressure medium is applied to the outside of the mould. The mould is supported by a metal cage or lantern ring which keeps the mould radially located, and distributes the fluid pressure medium evenly around the bag. A pressure of at least about 560 kg/cm.sup.2, preferably at least about 1760 kg/cm.sup.2, can be used to compact the powder, for example tungsten. Preferably, the radial pressure is applied for at least about 30 seconds to densify the powder to at least about 50 percent, preferably at least about 60 percent of theoretical density. Such compaction provides significant strength to the compact so that it can be easily handled and has sufficient structural strength to withstand the resistance sintering operation, without breakage.
The radial pressure applied to the mould by the fluid pressure medium is slowly removed in a controlled fashion to provide a controlled decompression of the compact. An uneven decompression of the compact may result in cracking or breakage of the compact. Pressure can be reduced at a rate up to about 70 kg/cm.sup.2 /second, preferably up to about 11 kg/cm.sup.2 /second to prevent such cracking or breakage of the compact.
The internal diameter of the void space in the mould is dependent upon the compaction ratio of the powder to be pressed. The compaction ratio and internal diameter are determined by means well known in the art to form a compact of the desired diameter. Such methods are described, for example, in "Tooling for Cold Isostatic Pressing" referenced above. For example, a tungsten compact of about 22 millimeters in diameter can be formed from a mould having an internal void space of 30.5 millimeters in diameter.
A tendency toward flaring of the ends of the formed compact has previously been minimized by careful adjustment of the inner surface profile in the mould. The punches sealing the ends of the cylindrical mould were provided to have a smaller diameter than the internal void space diameter of the cylindrical mould. The ends of the mould which seal around the punches are formed to correspond to the smaller diameter of the punches. The internal void space then has a small tapered section adjacent the punches tapering from the smaller diameter of the punches to the larger diameter of the void space.
The pressed compact is removed from the mould and sintered by resistance heating. As an example for tungsten, a resistance heating current of about 4,000 to 6,500 amps is transmitted through the compact according to cycles well known in the art. For example see "Application of Tungsten Wire as the Light Source in Incandescent Electric Lamps," D. J. Jones, Metallurgy and Material Technology, Volume 5 No. 10, pp. 503-512, 1973. Such resistance heating is sufficient to heat the compact to between about 2,100.degree. and about 3,000.degree. C. where the compact is sintered to about 85 percent of theoretical density. Other metals and ceramics compacts can be sintered by well known conventional methods.
However, solid rods of tungsten or other powder which are compacted by cold isostatic pressing typically have a cylindrical flared or "elephant's foot" region at each end due to friction between the punch and the powder. These elephant's feet have been found to generate significant yield reduction after either pressing or sintering; thus, indicating that "elephant's foot" is a major limitation in the efficient production of metal compacts by cold isostatic pressing.
Notwithstanding, the presentation of the art disclosed above, there continues to be a need for improved methods to eliminate or reduce cylindrical flared or "elephant's foot" regions at each end of powder, such as tungsten, compacted by cold isostatic pressing. Such methods desirably will provide increased product yield of both pressed and sintered articles while eliminating the need for the careful adjustment of the inner surface of the profile in the mould; would have a punch profiled in a geometric shape that would either eliminate or at least significantly reduce the elephant's foot region at least one end; would, in particular, reduce the diameter of at least one end region of the pressed compact during cold isostatic pressing; and would eliminate or at least significantly reduce the elephant's foot by having a taper applied at one or both ends of the cold isostatic pressed compact.